DNA-binding domain of Ecdysone receptor (ECR) family is composed of two C4-type zinc fingers. DNA-binding domain of Ecdysone receptor (EcR) family is composed of two C4-type zinc fingers. Each zinc finger contains a group of four Cys residues which co-ordinates a single zinc atom. EcR interacts with highly degenerate pseudo-palindromic response elements, resembling inverted repeats of 5'-AGGTCA-3' separated by 1 bp, upstream of the target gene and modulates the rate of transcriptional initiation. EcR is present only in invertebrates and regulates the expression of a large number of genes during development and reproduction. EcR functions as a heterodimer by partnering with ultraspiracle protein (USP), the ortholog of the vertebrate retinoid X receptor (RXR). The natural ligands of EcR are ecdysteroids, the endogenous steroidal hormones found in invertebrates. Like other members of the nuclear receptor (NR) superfamily of ligand-activated transcription factors, EcRs have a central well conserved DNA binding domain (DBD), a variable N-terminal domain, a non-conserved hinge and a C-terminal ligand binding domain (LBD).